This present invention relates generally to global navigation satellite systems (GNSS), such as the American global positioning system (GPS), the Russian Glonass system, and the European Galileo system. It relates specifically to synchrolites (i.e., frequency translators or rebroadcasters) for use with navigation signals from such GNSS systems.
GNSS systems can be used in a “stand-alone” mode to compute the position of a single antenna, or in a “differential” mode to compute the relative positions of two or more antennas. In conventional systems, each antenna is directly connected to a navigation receiver. It is occasionally desired to break this direct connection and separate the antenna from the navigation receiver. In such cases, the signals received by the antenna are transmitted over a separate communications link to the navigation receiver. This separate communications link has traditionally been a separate radio frequency, but it could alternatively be an acoustic or optical link (see U.S. Pat. No. 5,345,244). When a separate radio frequency was used, the original receiver or antenna side of the link is often called a “frequency translator” or “rebroadcaster.”
Frequency translators were first used in the early days of the American GPS program. In the late 1970's, various agencies desired to flight-test missiles and track them with GPS accuracy, but the existing GPS receivers were too large and heavy to fit inside a missile. The solution was a device (called a “GPS translator” at the time) which received signals in the GPS frequency band and retransmitted them in a telemetry frequency band from the missile to a navigation receiver on the ground, such as disclosed in U.S. Pat. No. 5,729,235.
These frequency translators had at least one local oscillator which is independent of the GPS satellite system clocks. The frequency translation is accomplished by mixing in a radio frequency mixer the local oscillator frequency with the signals received through the antenna, producing translated signals at a new frequency. The apparent frequencies of the GPS signals from the perspective of the navigation receiver on the ground are the sum or difference of the actual GPS frequencies with the frequency of the local oscillator on board the missile. To improve the navigation accuracy, a signal known as a “pilot tone” was added to the output of the translator in the same band as the translated GPS signals. The pilot tone was derived from the local oscillator in such a way that the respective frequencies had a fixed mathematical relationship. Spread spectrum pilot tones have been suggested. The navigation receiver on the ground then measured the frequency of the pilot tone, computed the frequency of the translator's local oscillator using the known mathematical relationship, and adjusted the received frequencies of the translated GPS signals to remove the effect of the translator's local oscillator.